I need to run a waste pipe from a new extension containing one kitchen sink. I have a surface water drain manhole on that side of the house but unfortunately the foul drain is on the other side of the house. I would have to run a pipe approx metres under existing suspended floor to access the foul drain stack. Can I use 40mm pipe if so what is the recommended gradient, or do I have to put in 75mm or100mm, I don't think there is sufficient space for 100mm. Any other suggestions?
Tom